The diminishing reserves of fossil fuels and the emission of harmful gases connected with their use have increased the interest in utilizing biological materials, especially from non-edible renewable resources for making liquid fuels capable of replacing fossil ones. Several prior art processes are known for producing liquid fuels from biological starting materials. One that has reached commercial success comprises the production of biodiesel (FAME) by transesterification of biomass-derived oils with alcohols.
Biofuel has also been successfully made from hydrocarbons produced from biomass gasification products via Fischer-Tropsch synthesis and from hydrocarbons produced by hydrodeoxygenation of triglycerides and fatty acids of biological origin. Furthermore, alcohols such as ethanol and methanol made from biological materials have been proposed for use as replacements for fossil fuels in combustion engines.
Methanol is the simplest one of the alcohols and it has the chemical formula of CH3OH. It is used as a solvent and as an industrial chemical in the manufacture of a wide range of raw materials including formaldehyde, methyl tert-butyl ether (MTBE), acetic acid dimethyl terephtalate (DMT), methyl methacrylate (MMA) methyl amines, antifreeze agents, etc. Methanol has also been suggested for use in the production of non-fossil fuels such as fatty acid methyl esters (FAME), dimethylether (DME), methanol-to-gasoline (MTG) and methanol-to-olefins (MTO). Furthermore, methanol has been proposed as a source of hydrogen for fuel cells.
Methanol is also called “wood alcohol” because it was previously produced as a byproduct of the destructive distillation of wood. It is now mostly produced synthetically by a multi-step process in which natural gas and steam are reformed in a furnace to produce hydrogen and carbon monoxide. The hydrogen and carbon monoxide gases are then reacted under pressure in the presence of a catalyst to form methanol.
Biomethanol, i.e. methanol of biological origin can be produced from various sources. It is typically produced by anaerobic digestion of biomass such as residues from various agricultural or forestry crops, waste products of animal and human effluents, municipal wastes and landfills, sugar beet pulp, glycerol etc.
Black liquor formed in the Kraft pulping process has been proposed as a source for production of biomethanol for use as non-fossil fuel. In this case, the black liquor is gasified to produce a mixture of hydrogen and carbon monoxide (synthesis gas) which is then converted into methanol.
Biomethanol is also produced as a direct by-product in the pulping of wood. In the Kraft pulping process, undesired side reactions of sodium sulphide with various wood constituents result in the formation of a large number of different organic sulphur compounds. In the evaporation of the black liquor, a condensate containing biomethanol is obtained. However, this biomethanol is contaminated with said sulphur compounds and has a very unpleasant odour. The contaminated methanol is traditionally incinerated in the mill to recover its energy content and to destroy the foul odour components.
Purification of black liquor derived methanol has been described in the prior art. Thus, U.S. Pat. No. 5,450,892 discloses a process for the scrubbing of black liquor condensate stripper off-gases. The alkaline scrubbing removes gases such as hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide and allows most of the methanol to remain in the scrubbed gases. The gases are then incinerated.
U.S. Pat. No. 5,718,810 discloses a process for the recovery of methanol from sulphur based wood-pulping processes using extractive distillation. According to the process, methanol is recovered from pulping process vapours which contain at least methanol and dimethyl sulphide. The vapours are distilled in two or three steps to provide methanol with a purity which may approach 100%.
In the production of hydrocarbon-based biofuels, the starting material is typically biomass which is gasified to provide synthesis gas or syngas. The synthesis gas is then led to a Fischer-Tropsch (FT) reactor to produce biohydrocarbons. Examples of suitable biomass sources include forest slash, urban wood waste, by-products and waste of the papermaking industry, lumber waste, wood chips, sawdust, straw, firewood, agricultural residue, dung and the like.
Gasifiers have been investigated for more than a century, and many different types have been developed. One drawback of biomass gasifiers still remains, however, namely their incapacity to produce a steady synthesis gas flow, having the optimum H2/CO ratio of about 2, to be utilized in the most effective cobalt based Fischer-Tropsch three-phase slurry synthesis. To correct the hydrogen to carbon monoxide ratio of the synthesis gas produced from biomass, make-up hydrogen is required.
Another possibility to produce hydrocarbon-based biofuels is to use biological triglycerides (bio-oils) or biological fatty acids (bio-acids) as starting materials. In order to make biofuel, the starting material is treated by a hydrodeoxygenation process (HDO). In the catalytic HDO process, hydrogen is combined with oxygen into water thus releasing the desired paraffinic biohydrocarbon backbone for chemical manipulation.
The subsequent phase in the biofuel production after the FT or HDO process comprises biohydrocarbon product upgrading. The upgrading processes typically comprise cracking and/or isomerization processes requiring hydrogen. Advantageously, one-dimensional molecular sieve catalysts such as Pt/Mordenite, Pt-SAPO's or Pt-ZSM-23,22 or equivalent are used to get a suitable, diesel range molecular length of the biohydrocarbon and to provide a side chain structure determining a desired cloud point and cetane value. Typically these upgrading processes employ relatively high hydrogen pressures without any significant hydrogen consumption.
Steam reforming of natural gas is the most common method of producing commercial bulk hydrogen. It is also the least expensive method. It is based on the idea that at high temperatures (700-1100° C.) and in the presence of a nickel-based catalyst, steam reacts with methane to yield hydrogen according to the equationCH4+H2O→CO+3H2 
Additional hydrogen can be recovered by a lower-temperature gas-shift reaction with the carbon monoxide produced.
Make-up hydrogen can also be produced from the synthesis gas obtained from the biomass and from tail gases of the biofuel fractionation. A standard procedure for providing more hydrogen is the well known water gas shift (WGS) reaction (CO+H2O→CO2+H2). However, the WGS reaction has its drawbacks. The WGS reaction is a catalytic process, hard to control and sensitive to synthesis gas impurities. Moreover, because the WGS reaction utilizes carbon monoxide, which is part of the synthesis gas, it lowers the total carbon conversion of the whole process scheme.
Thus, there exists a need for providing alternative sources of hydrogen for the production of biofuels. In order to provide a 100% biological and non-fossil fuel, a biohydrogen product is needed at reasonable costs. The biohydrogen should preferably be provided without utilizing carbon monoxide in a WGS reaction, since carbon monoxide is a component, which makes up the building blocks of the biohydrocarbon fuel. The present invention strives to satisfy that need. The present invention provides biohydrogen from a waste product of the pulp industry, namely from biomethanol.